Continuous tracks have been known for many years. They are extremely useful in situations where the ground is uneven and/or soft. The purpose of a continuous track is to spread the weight of the vehicle over a large surface area. This reduces the pressure exerted on the ground to a fraction of what the equivalent pressure would be if the vehicle had tyres instead of tracks. By reducing the pressure exerted on the ground the vehicle is less likely to become immobilised by poor ground conditions. As a consequence of this, the types of vehicles fitted with continuous tracks are often found on construction sites, mines, forests and other such areas where hard-surfaced, such as “metalled”, constructed roads are not present.
Continuous tracks are traditionally made up of several track segments connected together to allow, at least partial, relative movement therebetween. Alternatively, continuous tracks are known which are made from a continuous length of flexible material such as heavy-duty plastics and/or rubbers. These tracks may include gripping formations on their outer surface (these are sometimes known as “grousers” in the industry). These gripping formations may be chevron in shape, or be a series of ribs running perpendicular to the length of the track. Other patterns and shapes are also possible.
The tracks typically include engagement members on their inner surface for engaging with at least one drive wheel or sprocket. The continuous track runs partially around at least two wheels, at least one of which is a drive sprocket. Typically, for heavy-duty plant such as bulldozers or tippers, the track runs around three principal wheels such that the track forms an approximate triangular shape when viewed from the side. In this formation, the drive sprocket is typically upper most with the two lower wheels serving to define the shape but not necessarily being driven to drive the track. These vehicles are often referred to as “high-drives”.
It is also typical to have idler or jockey wheels associated with the continuous track. These help prevent the track from being pushed inwardly by uneven ground. In other words, they help to maintain the shape of the continuous track, especially along the lower, ground contacting, portion of the continuous track. Idler wheels may also be used at other points around the perimeter of the continuous track to maintain its shape for various reasons which will be discussed below.
With reference to FIG. 1, which depicts a known continuous track system 70, the continuous track 10 is held in an approximate triangular shape when viewed from the side by three wheels 20, 30, 40. The upper wheel 20 is the drive sprocket which drives the continuous track around the three wheels 20, 30, 40. The wheels 30, 40 are not driven in this case but merely act to maintain the shape of the overall continuous track 10. The three wheels 20, 30, 40 are maintained in approximate position to one another by means of sub-structure 50 which in this case is appropriately shaped steel work which maintains the centre points of each of the three wheels 20, 30, 40 relative to one another. The three wheels 20, 30, 40 all rotate about their centre points.
Idler or jockey wheels 60 are shown situated between the lower wheels 30, 40. These wheels 60 typically are provided in pairs. They help to maintain the track at the lower portion between lower wheels 30, 40 in contact with the ground.
For effective use and to prolong the life of the continuous track and its associated structure it is known that the tension in the track must be managed. In other words, a track with a reasonable degree of tension within it is better than one which is too slack. This is because a slack track increases the likelihood of slippage between the track and the drive sprocket. By contrast, a track which has too much tension in it is also undesirable since it can increase wear on the system and is inefficient in that there is more friction in the system which has to be overcome by the drive sprocket. With regard to the former undesirable position, namely too much slack in the track, it is known that continuous tracks may elongate over time due to wear. Accordingly, it is known to manage the tension in continuous tracks by the provision of a tensioner. Such tensioners may take the form of a spring urging one of the wheels away from the others. Other forms of tensioners are known such as hydraulic rams made up of a barrel, or cylinder, and a piston or piston rod. Such prior track tensioning systems are known from GB-A-2393696, JP-A-09240526, U.S. Pat. No. 4,874,052, EP2150458A and JP-A-61191982.
In FIG. 1 the axis about which wheel 40 may rotate is not fixed relative to the structure 50 about which wheels 20 and 30 rotate. Rather, its axis is linked via a tensioner 70 comprising a barrel 80 and a piston 90. By this arrangement, and by careful control of the hydraulic fluid within the barrel 80, the relative position of the axle for wheel 40 relative to the structure 50 may be adjusted so that in turn the tension within the track 10 is adjusted. This is because the piston may be extended or retracted relative to the barrel 80. In this way, the tension in the track may be increased or decreased as appropriate. However, it is typical for these tensioners to be set at a predetermined setting and for the settings to be only manually changed as and when necessary, for instance at times of servicing.
A disadvantage of the known system is that if an external force is applied to the continuous track 10 in the region of the tensioned wheel 40 the tensioner 70 may be overcome such that the piston 90 is pushed back in towards the barrel 80 thus making the track slack. In such circumstances it is known for the track 10 to become disengaged with either or both the drive sprocket 20 and forward guide wheel 30. Even when the external force is removed, the track may not re-engage with the wheels 20, 30 such that the vehicle is substantially immobilised until the track has been repositioned correctly. An example of an external force could be provided by the vehicle approaching an object on the ground surface such a rock.
Such tracked vehicles have a structure for supporting the tracks, the idler and jockey wheels. It is usually important to allow this structure to pivot relative to the vehicle body to allow for better maneuverability and to cope with uneven ground conditions. This structure is typically pivotably attached to the drive axle of the vehicle. This introduces stresses and strain on the axle which leads to premature wear and possible failure.
Accordingly, it is desirable to have an alternative continuous track system which provides a way of supporting it to reduce the stress and strain on the vehicle's drive axle and to maintain tension in the track.